[libvirt] [PATCH] qemu: allow larger discrepency between memory & currentMemory in domain xml

Laine Stump laine at laine.org
Fri Nov 16 19:01:46 UTC 2012

This resolves:


The reported problem is that an attempt to restore a saved domain that
was configured with <currentMemory> and <memory> set to some (same for
both) number that's not a multiple of 4096KiB results in an error like

  error: Failed to start domain libvirt_test_api
  error: XML error: current memory '4001792k' exceeds maximum '4000768k'

(in this case, currentMemory was set to 4000000KiB).

The reason for this failure is:

1) a saved image contains the "live xml" of the domain at the time of
the save.

2) the live xml of a running domain gets its currentMemory
(a.k.a. cur_balloon) directly from the qemu monitor rather than from
the configuration of the domain.

3) the value reported by qemu is (sometimes) not exactly what was
originally given to qemu when the domain was started, but is rounded
up to [some indeterminate granularity] - in some versions of qemu that
granularity is apparently 1MiB, and in others it is 4MiB.

4) When the XML is parsed to setup the state of the restored domain,
the XML parser for <currentMemory> compares it to <memory> (which is
the maximum allowed memory size for the domain) and if <currentMemory>
is greater than the next 1024KiB boundary above <memory>, it spits out
an error and fails.

For example (from the BZ) if you start qemu on RHEL6 with both
<currentMemory> and <memory> of 4000000 (this number is in KiB),
libvirt's dominfo or dumpxml will report "4001792" back (rounded up to
next 4MiB) for 10-20 seconds after the start, then revert to reporting
"4000000". On Fedora 16 (which uses qemu-1.0), it will instead report
"4000768" (rounded up to next 1MiB). On Fedora 17 (qemu-1.2), it seems
to always report "4000000". ("4000000" is of course okay, and
"4000768" is also okay since that's the next 1024KiB boundary above
"4000000" and the parser was already allowing for that. But "4001792
is *not* okay and produces the error message.)

This patch solves the problem by changing the allowed "fudge factor"
when parsing from 1024KiB to 4096KiB to match the maximum up-rounding
that could be done in qemu.

(I had earlier thought to fix this by up-rounding <memory> in the
dumpxml that's put into the saved image, but that wouldn't have fixed
the case where the save image was produced by an "unfixed"
 src/conf/domain_conf.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
index 9f2029b..2dd4490 100644
--- a/src/conf/domain_conf.c
+++ b/src/conf/domain_conf.c
@@ -8644,10 +8644,10 @@ static virDomainDefPtr virDomainDefParseXML(virCapsPtr caps,
     if (def->mem.cur_balloon > def->mem.max_balloon) {
         /* Older libvirt could get into this situation due to
-         * rounding; if the discrepancy is less than 1MiB, we silently
+         * rounding; if the discrepancy is less than 4MiB, we silently
          * round down, otherwise we flag the issue.  */
-        if (VIR_DIV_UP(def->mem.cur_balloon, 1024) >
-            VIR_DIV_UP(def->mem.max_balloon, 1024)) {
+        if (VIR_DIV_UP(def->mem.cur_balloon, 4096) >
+            VIR_DIV_UP(def->mem.max_balloon, 4096)) {
                            _("current memory '%lluk' exceeds "
                              "maximum '%lluk'"),

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